1. Field of the Invention
The invention relates to a power supply apparatus for a vehicle, which includes a main power supply that supplies electric power to a plurality of vehicle-mounted electric loads, and a subsidiary power supply charged with the electric power output from the main power supply, and a method of controlling the power supply apparatus.
2. Description of the Related Art
For example, an electric power steering apparatus generally includes an electric motor that provides steering assist torque to the turning operation of a steering wheel. The steering assist torque is adjusted by controlling the feeding of electric power to the electric motor. Such an electric power steering apparatus uses a vehicle-mounted power supply (a battery and a power generator) as a power source, and consumes large electric power. Therefore, for example, an apparatus proposed in Japanese Patent Application Publication No. 2007-91122 (JP-A-2007-91122) includes a subsidiary power supply that assists the vehicle-mounted power supply. The subsidiary power supply is connected in parallel to a power supply line that extends from the vehicle-mounted power supply (hereinafter, referred to as “main power supply”) to a motor drive circuit. The subsidiary power supply is charged with the electric power output from the main power supply. The subsidiary power supply supplies the electric power to the motor drive circuit using the electric power with which the subsidiary power supply is charged.
The apparatus proposed in the publication No. 2007-91122 includes a power-feeding switch that allows and prohibits the feeding of electric power from the subsidiary power supply to the motor drive circuit. When target electric power used to operate the electric motor is larger than a predetermined value, the power-feeding switch is turned on to form a power supply circuit that supplies the electric power from the subsidiary power supply to the motor drive circuit. Also, the apparatus includes a charging switch that allows and prohibits charging of the subsidiary power supply with the electric power output from the main power supply. When it is determined that the subsidiary power supply is not in a full-charge state based on a voltage between both terminals of the subsidiary power supply, the charging switch is turned on to charge the subsidiary power supply with the electric power output from the main power supply.
In the apparatus proposed in the publication No. 2007-91122, the subsidiary power supply is charged when the voltage between the both terminals of the subsidiary power supply is lower than the reference voltage. That is, the subsidiary power supply is charged simply based on the voltage between the both terminals of the subsidiary power supply. Therefore, an effective charging control according to the traveling state of a vehicle is not executed.